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Original question:

UCC28070: About Current Sense Resistor.

UCC28070: How high speed op-amp is needed for resistive sensing

Kazuki Itoh
Genius 10760 points
Part Number: UCC28070

Hello,

I'd like to know how fast op-amp I need to pick up for UCC28070 resistive sensing.

I'm interested in how you picked up 10MHz op-amp for 1MHz PFC on previous thread.
Considering low duty cycle at 265 Vin 400 Vout, current pulse should be very narrow, so it seems higher speed op-amps are needed.
1401.Sch-1.pdf

  • 0
    TI__Mastermind 34465 points
    Yes you are correct.
    The design you attached has a 100kHz switching frequency so a 10MHz opamp should meet any high speed requirements.
    The UCC28070 will not operate at 1MHz unfortunately.

    Regards
    John
  • 0 in reply to John Griffin1
    TI__Genius 10760 points
    Thank you very much.

    Assuming maximum input voltage is 265VRMS and output voltage is 400VDC, minimum duty cycle will be
    Dmin = (400V - 265V*1.414)/400V = 0.063
    So, minimum on-time will be
    0.063 / 100kHz = 630ns

    This circuit uses 10MHz op-amp and the gain is 5.62, so bandwidth should be 1.78MHz.
    Also, this circuit has low-pass filter with RC=220ns.

    Does it meet any high speed requirements?

    Regards,
    Itoh
  • 0 in reply to Kazuki Itoh
    TI__Mastermind 34465 points
    Hello Itoh,
    Why dont you just test this idea with your square wave input to the OPAMP and see what the output signal looks like?
    If you think it is acceptable then you can make a decision as to whether to use this new information.

    Regards
    JOhn